Art of esterification



Patented Oct. 28, 1947 UNITED STATES PATENT OFFICE ART OF ESTERIFICATIONWilliam Beach Pratt, Boston, Mass. Annette Harris Pratt, administratrixof said William Beach Pratt, deceased, assignor, by mesne assignments,to Joseph G. Denny, Jr.

N Drawing. Application August 11, 1939, Serial N 0. 289,712,

7 Claims.

hydrate, such as dehydrated cellulose, with a substantially anhydrousalcohol, such as methanol, containing an esterification catalyst, suchas a perchloric acid hydrate, forming a relatively fixed constantboiling acid soluble in methanol, and which is convertible bydehydration into a volatile oxide which is more effective as anesterification catalyst than the mineral acid itself.

The impregnation of the carbohydrate with alcohol places it on the samephase of reaction;

as a suitable substantially anhydrous dehydrating and esterifyingreagent, such as acetic anhydride. The anhydride reacts with the hydrouscatalyst to form a volatile and more effective catalyst,

such as chlorine heptoxide; reacts with the alco-,, hol to form waterand a diluent of anhydridej such as methyl acetate, and reacts rapidlybut uniformly with the carbohydrate to form an ester, such as acetylcellulose. The catalyst thus formed in situ in the carbohydrate isunstable in the; presence of oxidizable material, such as cellulose,

and-is stabilized during esterification of the carbohydrate andthereafter evaporated from the ester, which consequently requires littleor no washing to prevent degeneration. The stabilization of the unstablecatalyst is preferably effected by maintaining oxygen or air in contacttherewith.

Acetyl cellulose may be produced from natural fibres of normalcellulose, such as cotton or bast, fibres, by my method and retains thetoughness,'

flexibility, elasticity, strength and uniformity, as well as thephysical form, configuration and convolutions of the natural fibres. Bynormal cellulose, I mean cellulose which has not been degenerated byhydrolysis or oxidation or regenerated from solution. Whilehydrocellulose, oxycellulose and regenerated cellulose are more readilyesterified than normal cellulose, such celluloses and the esters thereoflack the toughness, flexibility,

elasticity and strength ofnormal cellulose and esters made therefrom bymy process. Yarn suitable for weaving or knitting may be spun entirelyfrom acetyl cellulose fibres made from normal cellulose by my method.

My method also permits the esterification of natural fibres of normalcellulose after they have been spun into yarn or woven into fabric andresults in an increase in the diameter and number of turns per inch inthe yarn and the translucency or transparency of such yarns or fabricmay be largely controlled by the tension applied thereto duringesterification. Both the yarn and fabric have as much or more toughness,fiexi-- bility, elasticity and strength as similar yarns or fabricsof'untreated normal cellulose, and possess these qualities in muchgreater degree than yarns or fabrics made from hydrolized, degenerated,or regenerated cellulose either before or after esterification thereof.

My method of esterification avoids the localized reactions resulting inan irregular rate of esterification, accompanied by high temperaturesand disintegration or possible degeneration of the cellulose,characteristic of esterification as heretofore practiced by reactionbetween cellulose material and esterifying acids in the presence of anesterification catalyst and unevenly distributed water. My inventionobviates the need for the exacting thermal control ordinarily requiredin carrying on esterification.

My invention facilitates and unifies the esterification without thedegeneration or degradation of the cellulose molecules beforeesterification, or the solvation or disintegration of the cellulosecells or fibres during or after esterification, and which result in lossof toughness, flexibility, elasticity, strength and uniformity in theresulting product as compared with the characteristics of natural fibrescomposed'of normal cellulose.

My improvements moreover eliminate the need for long after treatments ofacetylated products for their hydrolization, purification, and removalof the esterification catalyst or reaction products thereof.

My improvements further provide esterified cellulose in the form oftextiles or yarn composed entirely of esterified cellulose staple, whichhas been heretofore unknown, or in the form of a celluloseesterification.

3 liquid or plastic, which may be used without dissolution thereof inexpensive solvents.

My improvements further permit the formation of yarns or textiles ofvarious and contrasting colors or shades by dyeing or printing thecellulose fibres, yarns or fabrics with ordinary cotton dyes (such asthose which are set or developed by acids), prior to the esterificationthereof, thereby eliminating or minimizing the need of the special andexpensivedyes ordinarily required for dyeing cellulose acetate.

I have discovered that localized and irregular reactions duringesterification are due primarily to irregular distribution of the wateroccluded in, or physically or chemically combined with-the cellulose orthe esterifying bath. While the presence of water is essential ,to theesterification of cellulose, I have found that its .action can be.effectively controlled only by the formation thereof by reaction in situand that all the substances involved in the esterification-should beinitially so anhydrous that their aggregate water content is initiallyinsufficient to initiate esterification of the cellulose or localreaction.

The practice of my method therefore requires the exclusion of water tothe utmost degree possible from the esterifying substances and theuniform diffusion of any minute quantity of such extraneous'water thatmay remain; and the generation,.by reaction, of Water in situ in themolecular structure of the cellulose sufficient to activate Such waterof reaction is generatedbythe esterification of a water soluble alcoholwhich more readily esterifies than cellulose; such preliminaryesterification being effected immediately prior to orconcurrently withthe .esterification. of the cellulose and in intimate contact with themolecular structure thereof.

fI'heremoval of all free Water, including physically or chemicallycombined water, from the cellulose and the complete impregnation of itsmolecular structurewith alcohol places the cellulose upon. the samephase. of reaction as the waterfree esterifying bath. By thuseliminatingany interfacial tension between the constituents of the esterifying bathand'the products to be esterified, the latter are completely, uniformlyand substantiallyinstantaneously penetrated and impregnatedby the'formerso that the esterifying reaction takes place uniformly throughout the'50 molecular structure.

I have further found that the mineral acids, such as. sulphuric acid andphosphoric acid, ordinarily used in practice as esterificationcatalysts, so combine with cellulose-that their removal entails long anddifficult washing operations which result in an after-degeneration ofthe cellulose, and should any such acids remain in the fabricityresults-in an ultimate weakening of the fibre or-ageing.Esterification is greatly accelerated by the use of chlorine heptoxide(C1207) as the esterification catalyst. Such chlorine .heptoxide hasnoinjuriousafter effects since it readily and completelyvolatilizes'fromthe cellulose after the esterification thereof iscompleted. The chlorine heptoxide is preferably formed in situ inthe'cellulose and its tendency to instability may beovercome bymaintaining oxygenin contact therewith during esterification. Theformation of thechlorine heptoxide in situ is preferably effected by 70the reaction of an esterifying anhydride or of phosphorous pentoxide ona perchloric acid hydrate containing '70 to 12% .of H0104 and twomolecules of water, and which is capable of holdgerous and harmful.

"weight ofhygroscopic -72% perchloric acid.

There is no apparent reaction of the perchloric acid on the methanol oron the cellulose in this treatment even at the boiling point ofmethanol.

The-methanol uniformly disperses the catalyst throughout the cellulosefibres and removes or unifies the distribution of the last traces ofwater moisture from thecellulose, but does not remove the water ofhydration from the perchloric acid.

The saturation point of the cellulose fibres is about 62% 'of theirweight of methanol and about l'.5 of their weight of perchloric acid,which is not selectively absorbed from the methanol. Such a highmethanol content is unnecessary and generally undesirable foresterification, since the amount of methanol carrying the catalyst leftin the fibres should only be enough to form by reaction with anhydridesufficient water for esterification of the cellulose. Consequently thesaturated cellulose, after being passed through squeeze-rolls,-is drieduntil the methanol content is reduced by evaporation to *between 10% and30%, and preferably about 20% of the weight of the cellulose. Theperchloric acid content is un- "affected by the drying and remains about1.4% to 1.5% of the weight of the cellulose. The cellulose so dried isimmersed in or covered by an esterifying bath, such as acetic anhydridealone or mixed with a diluent,such as anhydrous methyl or ethyl acetate,which is a non-solvent for cellulose acetate.

The penetration of the acetic anhydride is immediate and uniformthroughout the molecular structure of'thecellulose and the reactiontakes place progressively to form a mono-, di-, or triacetate accordingto the proportion of anhydride present to the cellulose and methanol.

The perchloric'acid is dehydrated by the acetic anhydride to formchlorine heptoxide (C1207) and. water; the anhydride entering thisreaction only so far as to pick up the water from the perchloric acidand form acetic acid.

The amount of water released by the dehydration of the perchloric acidis negligible, but the reaction between the anhydride and the methanolin the fibres, in the presence of the catalyst, forms methyl acetate andwater in situ in the fibres. This water converts a portion of the aceticanhydride to acetic acid and activates the acetylation of the cellulosein the presence of the chlorine heptoxide (anhydride of perchloric acid)which acts as the catalyst to the esterification of the cellulose.Normally chlorine heptoxide would break down in the presence ofcellulose and give off chlorine dioxide (C102) and unknown oxychlorinecompounds, which are dan- I prevent this breakdown of the chlorineheptoxidein the acetylating solution or in the cellulose by maintainingair containing oxygen in contact therewith.

The requisite contact of oxygen with the catalyst during esteriiicationmay be maintained by working in the open air, or blowing air onto thecellulose fibres where only the necessary amount ing 1.5 more moleculesof water. Such perchloric ofacetic anhydride is used. Where a diluent isadded to the anhydride and the cellulose is immersed in the bath duringesterification, air should be blown through or occluded in the solution,as for instance by agitation thereof in closed containers. Themaintenance of contact of oxygen with the catalyst during esterificationand the subsequent evaporation of the catalyst is essential to securingthe highest values in the esterified cellulose.

The ultimate volatilization of the chlorine heptoxide from the celluloseacetate leaves the latter free from any injurious mineral acids orresidues such as are left when other inorganic acids are used as thecatalyst. This renders possible the complete removal of any reagentsleft in the acetylated fibres by a wash in cold water for about fiveminutes.

While the best and most uniform results are obtained by the use as anesterification catalyst of chlorine heptoxide formed and stabilized insitu from perchloric acid diffused in the cellulose by the methanol, itwill nevertheless be understood that the catalyst may be introduceddirectly into the acetic anhydride, and that small quantities of mineralacids, such as sulphuric, perchloric or phosphoric acids may be used ascatalysts to cellulose esterification, in conjunction with novel stepsherein described, if long washing, hydrolization, and less perfectproducts are tolerable.

When sulphuric acid is used as the esterification catalyst, 1.07% ofsuch acid by weight relative to the weight of methanol has been foundgenerally sufiicient, but additional amounts up to say 3 may be usedwithout showing excessive ill effects when the methanol content of thecellulose is of the order of 25% by weight of the cellulose and theesters are long and thoroughly washed.

In lieu of methanol, other anhydrous but water soluble alcohols, whichreact with esterifying acids to form water and in which cellulose andcellulose esters are insoluble, may be used, as, for instance, othermono-hydric alcohols, such as dehydrated ethyl or butyl alcohol,di-hydric alcohol, such as glycol, and tri-hydric alcohol, such asglycerol. The hygroscopic nature of ethyl alcohol, however, renders moredifficult the control of the reactions because of the tendency of ethylalcohol to carry water into the fibres. Butyl alcohol and the like alsotend to absorb water and even when free from water are slow ininitiating reaction on introduction of the cellulose into theesterifying bath, and when the reaction occurs it is violent and causesa greater rise in temperature than where methanol is used. Glycerol andglycol are also generally less satisfactory than methanol because thereaction in the esterifyin bath lacks the uniformity of effect that isproduced where methanol is used.

In lieu of acetic anhydride, other esterifying anhydrides or anhydrousfatty acids may be used such, for instance, as formic acid.

My method permits the use of amounts of esterifying acids or anhydridesso close to the amounts theoretically required for reaction with thealcohol and with the cellulose that uniform mono-, di-, or tri-esters ofcellulose may be directly produced by properly proportioning the amountof the esterifying anhydridev or acid to the alcohol and cellulose,thereby avoiding the need of hydrolizing or saponifying the primaryesters to secondary esters.

The factors governing the percentages of acetic anhydride necessary forthe formation of a desired primary or secondary cellulose acetate, thereactions involved, and controls desirable in commercial practice areillustrated by the following table applicable to the introduction intoacetic anhydride of pounds of cellulose fibres containing 30% ofmethanol and catalyst as hereinbefore set forth:

Pounds of water formed 8.46 Pounds of methyl acetate formed 69.3 Poundsof 100% acetic anhydride hydrolyzed by the water formed 48.0 Pounds ofacetic acid formed from the hydrolysis of the acetic anhydride 56.4

To form cellulose triacetate under such conditions there shouldtheoretically be initially present 293 pounds of acetic anhydride; or toform cellulose diacetate there should be initially present 227.3 poundsof acetic anhydride; or to form cellulose monoacetate there should beinitially present 161.6 pounds of acetic anhydride. In commercialpractice it is desirable to use approximately ten per cent in excess ofthe amounts of acetic anhydride theoretically indicated.

The uniform saturation of fibres with such small amounts of anhydride isdiificult, hence it is generally desirable to add to the anhydride asuflicient quantity of diluent to facilitate and unify reaction. Whenthe cellulose is simply passed through the esterification bath and thereaction completed during the movement of the cellulose through the air,only about one half as much diluent is required as when the cellulose iscompletely esterified in the bath.

The diluent used may be varied to meet variations in the form orcondition of the cellulose or variations in the impregnating alcohol andin the esterification catalyst, variations in the equipment used, orvariations in the product desired. The most advantageous diluent for thepractice of my method under any given set of conditions is readilydeterminable by empirical tests, but they should be water insoluble andfree from water, inert with respect to the impregnating alcohol, theesterifying acid and to the esterification catalyst, and when fibrestructure is to be maintained the diluent should be inert to thecellulose ester formed. When the cellulose fibres to be treated have notbeen degummed it is desirable that the diluent, as well as theimpregnating alcohol, be a solvent for the natural gums and waxes of thefibre.

Illustrative examples of suitable diluents are methyl acetate, toluol,benzol or xylol. The esterifying bath may also include or contain heavymineral oils and coal tar derivatives which have no catalytic action butvary the physical characteristics of the fibres and preserve oraccentuate desirable characteristics in the esterified fibres.

When it is desirable to form a cellulose ester solution rather thanmaintain the identity of the fibres, the diluent may comprise or containa solvent for the particular cellulose ester being formed, as forinstance, acetic acid may be used as the diluent when a solution ofcellulose acetate is to be formed.

The surface only of a cellulose fabric may be dissolved as an incidentof its conversion into cellulose acetate so as to provide a bindersecuring pile threads therein or other fabrics thereto.

When using perchloric acid, to the ultimate formation of chlorineheptoxide, as an esterification catalyst, methyl acetate (99.5% pure)was found to be the most satisfactory diluent for the acetic anhydrideand to produce the most satisfactory results when solvation of thecellulose acetate was not desired. This diluent may be economicallyrecovered due to its low boiling point and solubility to a degree inwater.

Methyl acetate is not, however, a desirable dilucut for acetic anhydridewhere sulphuric acid is used as an esterification catalyst, and in suchcase toluol may be successfully employed as a diluent.

In the practice of my invention for the continuous production ofcellulose acetate fabric, a cotton fabric, which has been degummed by ausual kier boil and bleached to produce standard or normal cellulose ofgreat purity substantially free from degradation products, has its freeand combined moisture evaporated therefrom by passage through a suitablyventilated section of a tcnter drier having a temperature sufficient toextract all the moisture but insufficient to cause any appreciableoxidation or decomposition of the cellulose. When the moisture contentof the fabric has been reduced below 1%, the dehydrated fabric passesfrom the drier section through a bath consisting of anhydrous methanoland perchloric acid and is thoroughly impregnated therewith. Themethanol and acid content of the soaked fabric may be reduced by passageof the fabric between squeeze rolls, and the methanol content of thefabric is further reduced substantially below itssaturation point bypassing the fabric through a further drier section until the methanolcontent has been evaporated down to between and 30% and preferably 20%of the weight of the goods and the acid content is approximately 1 /2%of the weight of the goods. The evaporated methanol may be recoveredfrom the dryer and condensed.

The fabric is cooled by evaporation of methanol therefrom and is fedthrough a solution containing 100 parts of acetic anhydride and 50 partsof methyl acetate. It is then passed, under tension, over rollers, incontact with a stream of air. The acetylation is completed in air andthe chlorine heptoxide evaporated from the cellulose acetate in aboutten minutes. The temperature of the bath does not require externalthermal control as the rise in temperature is insufficient to degeneratethe cellulose, and when the bath contains diluent equal to several timesthe weight of anhydride the temperature rise of the bath is negligible.The fabric is then fed into a washer and washed in water at atemperature not exceeding the estcrification temperature. Ordinarily anyresidual reagents may be completely washed out within ten minutes withwater at a temperature of 60 to 80 F. The product is then in suitablecondition for finishing as. a textile fabric.

Having described my invention, I claim:

I 1 In the art of acetylating cellulosic material, the steps whichinclude dehydrating the cellulosic material below its normal water ofcondition; impregnating the cellulosic material while so dehydrated witha liquid impregnator comprising amixture of anhydrous methanol and astable perchloric acid hydrate having a constant boiling point wellabove the boiling point of methanol; subjecting the cellulosic material,while impregmatedwith between 10 and 30% of methanol on the weight ofcellulosic material and with said perchloric acid hydrate to the actionof an acetylating bath containing acetic anhydride more than sufficientto both remove the water of hydration from the perchloric acid hydrateand to molecularly combine with the methanol present;

the aggregate of free water in the cellulosic material, in theimpregnator, and in the bath being insufiicient when they are initiallybrought together to initiate appreciable acetylation of the cellulosicmaterial by the anhydride in the absence of the methanol; and continuingsuch action for a time and at a reacting temperature, effectingrelatively rapid reaction between the anhydride and cellulosic materialwithout appreciable degradation of the latter.

2. In the art of acetylation, the steps which includesubjecting normalcellulose substantially freefrom degradation products to a dehydratingtemperature insufficient to cause appreciable oxi" dation or degradationof the cellulose; impregnating the fabric when its moisture content isreduced below 1% with an impregnator which is substantially inertrelatively thereto and comprising a liquid mixture of anhydrous methanoland perchloric acid hydrate; volatilizing from the fabric a portion ofthe methanol therein at a temperature below the boiling point of theperchloric acid hydrate until the methanol content is between 10% and30% of the weight of the goods; subjecting the so impregnated celluloseto the action of an acetylating bath comprising acetic anhydride andmethyl acetate for less than an hour at an acetylating temperaturewithout appreciable degradation of the cellulose; andremoving anyresidual reagents.

3. In the art of acetylating cellulose, the steps which includeuniformly dispersing in the cellulose perchloric acid by impregnatingsuch cellulose with a perchloric acid hydrate dissolved in solventtherefor, and subjecting the impregnated cellulose to the action of anacetylating bath containing acetic anhydride in excess of that requiredto remove the water of hydration from the perchloric acid hydrate andform chlorine heptoxide and in contact with sufficient oxygen tomaintain the stability of the chlorine heptoxide so formed.

4. The method of acetylation which comprises impregnating substantiallyanhydrous cellulose with substantially anhydrous methanol containingperchloric acid, treating the impregnated cellulose with sufficientacetic anhydride to form chlorine heptoxide by reaction with the acid,to form water by reaction with the methanol. and to form acetyls byreaction with the cellulose at reacting temperatures, stabilizing thechlorine heptoxide by oxygen during acetylation, and permitting thechlorine heptoxide to volatilize from the esters.

5. A process for the manufacture of cellulose esters which comprisesconducting acetylation of the cellulose in contact with chlorineheptoxide and oxygen sufficient to stabilize the chlorine heptoxide.

6. In the art of acetylating cellulose, the steps which includeuniformly dispersing in the cellulose a perchloric acid hydrate having asubstantially constant boiling point dissolved in solvent therefor,converting said hydrate while dispersed in the cellulose into chlorineheptoxide by the removal of water of hydration therefrom by the actionof acetic anhydride thereon, acetylating the cellulose by the action ofacetic anhydride thereon during the dehydration of said hydrate, andstabilizing said chlorine heptoxide during acetylation of the celluloseby supplying oxygen thereto.

7. A step in a process for the manufacture of cellulose esters whichcomprises conducting acetylatlon of cellulose in contact with chlorineheptoxicle and oxygen free to gombine therewith.

WILLIAM BEACH PRATT. REFERENCES CITED The following references are ofrecord in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,236,578 Lindsay Aug. 14, 19172,008,021 Kenety July 16, 1935 2,064,384 Richter Dec. 15, 1936 2,087,036Malm July 13, 1937 2,045,161 Muller June 23, 1936 Number Number

